The present invention relates to the dewatering of sludges, slurries, or the like, that contain macro-molecules, colloidal particles, and/or suspended particles in a carrier liquid, usually water, and, more particularly, to dewatering of such materials using electrokinetic techniques including both electrophoretic and electro-osmotic techniques.
Many industries, especially the paper making and related fiber production industries, generate large quantities of sludge or slurry-like effluents that contain various concentrations of materials generically described as "solids" that can include, for example, dispersed and semi-dispersed macro-molecules, colloidal particles, and suspended particulate matter. As part of the treatment of the effluents, the liquid carrier or water is separated from the solids with the so-separated water being recycled to the industrial process or returned to the environment and the solids disposed of through landfill operations.
In the past, the dewatering of effluents to isolate solids has been accomplished by thermal evaporation, mechanical filtration, chemical processing, settling, flotation, and variety of other methods. In some special cases, electrical dewatering of effluents and sludges has been accomplished by establishing an electric field in the sludge between electrodes immersed within the sludge. As a consequence of the applied electric field, particles within the sludge that carry a net positive or negative charge, including macro-molecules, colloidal particles, and suspended particles, electrophoretically migrate to their respective, oppositely charged, counter electrode. In addition to the migration of the particulate matter, water molecules which are bound to or otherwise associated with some of the charged particles will co-migrate with those particles. As a consequence of this migration of charged particles and associated water molecules, the solid particles consolidate and densify to effect partial separation from the liquid carrier. Filtration devices can be provided at or near one of the electrodes to permit removal of the water by electro-osmotic techniques. With continued application of the electric field and water removal, the solids concentration of sludge increases to the point where the solids densify and consolidate into a load-bearing material to complete the dewatering.
The principal economic cost of electrokinetic dewatering has been the cost of electricity for generating and sustaining the applied electric field. In the past, direct current has been applied to the electrodes to establish the electric field. Direct current has the advantage of establishing a uniform field but has the associated disadvantage of not being readily available from power mains and, accordingly, must be generated on site, usually through motor-generator sets. Experience indicates that the dewatering rate, that is, the rate of water removal over time, or conversely, the rate of solids consolidation, is roughly proportional to the power consumed in kilowatt hours (KWH). The economic costs associated with dewatering an acre of sludge as part of a landfill operation are presently between $8500-$9500 (1980 dollars). Accordingly, there is an economic need to effect dewatering by electrokinetic techniques at substantially reduced electrical energy costs.
In addition, there is a continuing need in the art of electrokinetic dewatering to dewater sludges continuously in a rapid, cost-effective manner, in situ, to ease the burden of sludge handling and disposal of material reclamation. In addition, it is desirable to be able to adapt existing conventional continuous sludge conveyances readily to utilize the techniques of this invention. It is toward the solution of these aspects of the problems that this application is directed, as well as to the application of the techniques described in the parent case to a continuous dewatering system.